Method for preventing the corrosion of distillation equipment



Patented Sept. 24, 1946 UNITED STATES PATENT OFFICE METHOD FORPREVENTING THEOORRO- SIGN OF DISTILLATION EQUIPMENT David C. Walsh, Jr.,and Elza Q. Camp, Goose Creek, Tex., assignors to StandardOilDevelopment Company, a corporation of Delaware No Drawing.Application April 27, 1944,

1 Serial No. 533,064

4 Claims. 1

which boils substantially within the gasoline range. Generally thealkylation reaction is carried out in the presence of concentratedsulfuric acid. The sulfuric acid and alkylate recovered from the reactorare usually discharged into a separation vessel where the sulfuric acidand the alkylate are permitted to stratify. From the separation vesselthe sulfuric acid is withdrawn as a bottoms and recycled to the reactionzone. The alkylate, on the other hand, is taken off overhead, washedwith caustic soda solution, then washed with water and then pumped to acharge tank from which it is subsequently withdrawn and charged into adebutanizer column. The alkylate is then charged to a fractionating zonewhere the butane fraction and the octanes and lighter hydrocarbons inthe case of butylene alkylate or the pentane fraction and octanes andlighter hydrocarbons in the case of pentylene alkylate are removed. Thebottoms from this operation are then further reduced in a still forremoval of any fractions suitable for use in motor gasoline but too highin boiling range for inclusion in aviation fuel.

The foregoing operation results in corrosion in the alkylatefractionating columns, in the reflux accumulators, in the transfer linesto the fractionating equipment, and in the charge storage tanks, but themajor corrosion has been found to occur in the fractionating columnpreheaters and reboilers.

An aqueous caustic (2%5% caustic) solution wash followed by a water washhas been employed for the elimination of $02 in the alkylate productgoing to the distillation unit charge tank. However, the use of causticand water wash serve only to remove free S02 and entrained acid, but hasnot prevented the corrosion and fouling tendency of the alkylate duringsubsequent handling and processing. It is believed that decomposition ofesters in the preheater and reboiler system results in the formation ofdeposits and volatile acidic materials, such as S02, since tests have 2indicated that heating the alkylate product to temperatures comparableto those used in the fractionating unit reboilers Will cause S02 to beliberated along with a tarry'material which is deposited. The productionof this unstable material is not an inherent characteristic of thealkylation reaction but is probably the resultof side reactions varyingwith operating conditions since sometimes the decomposition effect isvery severe and at other times almost entirely negliible.

The injection of phenolic bodies such as tricresol or crude petroleumphenols into the alkylate is disclosed in pending application U. S.

Serial No. 417,275, filed October 31, 1941, for John L. Hart which hasissued as Patent No. 2,393,531, Jan. 22, 1946. The present inventionrepresents an improvement over the aforementioned application in whichphenolic products are injected into the alkylate.

In the present invention we have discovered that the corrosivity andfouling tendency of the alkylate, by compounds released therefrom by theheat treatment during the distillation, may be substantiallyeliminated-by injecting into the alkylate an alkaline inhibitor,specifically an alkaline compound of a phenol. The amount of alkalinecompound to be injected may be varied over a fairly wide range butgenerally will include amounts from .005 to .5% by volume, based on Jthe total alkylate being treated.

In the injection of the alkaline inhibitor, it is preferred that thecompound be introduced into the system at a point where the alkylate isflowing in a relatively turbulent stream. If the inhibitor is injectedinto the alkylate stream prior to distilling, and is allowed to come incontact with water and to remain in a quiescent state, it is likely thatthe beneficial effects of the inhibitor will be lost due to theinhibitor dissolving into the water phase rather than remaining incontact with the hydrocarbon phase. Consequently, we prefer to injectour inhibitor into the alkylate immediately prior to the alkylate entryinto the distillation equipment.

The reasons for the unexpected benefits of our invention are notcompletely understood but it is believed to be a dual effect resultingfrom the alkalinity of the phenolic salt as well as the phenolic groupitself. For example, in the aforementioned pending application, on whichthe present invention represents an improvement, the phenolic compoundsare known to be distilled ofi and to be carried out with the alkylate.This is because of the appreciable vapor pressure of the phenols in thealkylate. On the other hand, the alkaline inhibitor of the presentinvention, which may be the sodium salt of a phenol, for example, has arelatively low vapor pressure and consequently remains in the bottom ofthe distillation tower with the heavy fractions Where it exerts itsbeneficial effects. The sodium ion is believed to react with the acidicvapors at the point where they are released from the hydrocarbonsolution, and the phenolic group is believed to exert an inhibitingeffect in protecting the metallic surfaces from corrosion and also fromdeposition of the fouling bodies.

It is to be understood that any high boiling phenolic compound, such asa mono or polyhydric phenol after being converted to an alkaline salt issuitable in the practice of the present invention. cresol, a mixture ofortho-, meta-, and paracresol, pyrogallol, resorcinol and hydroquinone.Another phenolic material, which we prefer to use in the practice of thepresent invention, is sodium salt of phenolic compounds obtained frompetroleum. These phenolic compounds obtained from petroleum are commonlycalled petroleum phenols, and the methods of extracting "them frompetroleum fractions are well known and further mention need not be madehere. It will sumce to say that the sodium salts of the petroleumphenols are quite effective in eliminating the aforesaid fouling andcorrosion difiiculties. These types of compounds are particularly usefulsince, besides the effect of the sodium ion and the phenolic group, itis believed that the petroleum phenols contain natural inhibitorsExamples of such materials are tri-' the spirit and scope of the presentinvention to use various fractions of the petroleum phenols and then toreact the various fractions with the alkali metals to obtain the desiredsalt for use in the practice of the present invention. Usually it willbe preferredto use the high boiling phenols for conversion to thecorresponding alkali metal against corrosion of metals by acidiccompounds appreciably higher than the boiling point of the V heaviestalkylate fraction, it is possible to separate the inhibiting materialfrom the alkylate by distillation.' The type of oil employed as afluxing medium may vary considerably but generally it is preferred touse an oil in the gas oil or light lubricating oil boiling range. It isto be understood, of course, that the oil used as a fluxing mediumpreferably should be highly refined and should not in itself contributeany deleterious bodies to the alkylate.

In the present invention the use of sodium salts of phenolic compoundshas been mentioned. It

. is to be understood, of course, that other alkali metals may besubstituted for the sodium salt employed in the present invention. Thesodium salt of the phenolic compounds may be obtained, particularly inthe case where petroleum phenols are employed, by interrupting one ofthe steps salts.

Th invention will be further illustrated by the following example.Alkylate produced by the sulfuric acid alkylation process from isobutaneand butylene was treated in accordance with conventional methodsincluding the step of distilling the alkylate in a fractionating column.It was found that the lower portion of the fractioning tower was beingseverely corroded by the alkylate and corrosion continued even afterphenolic bodies were injected into the tower in accordance with theteaching of the Hart application, Serial No. 417,275. The operation wasthen altered by introducing a sodium salt of petroleum phenols into thealkylate being distilled in the fractionating column in accordance withthe present invention, After this change in the operation, inspectionsof the fractionating-tower showed that the corrosion had beensubstantially eliminated and that the tower was free from material whichformerly deposited on and fouled the heating surfaces. The sodium saltof petroleum, phenols was added to the alkylate by making up a solutionof sodium salt in an equal volume of lubricating oil distillate; thissalt was added to the alkylate in amounts corresponding to .025% byvolume of the sodium salt based on the total alkylate, This solution didnot affect the quality of the alkylate, produced as the overheaddistillate, to-any extent and it materially reduced the corrosion andfouling difficulties. 4

The nature and objects of the present invention having been completelydescribed and illustrated, what we wish to claim as new and useful andto secure by Letters Patent is: 1. In the distillation of hydrocarbonswhich have been contacted with sulfuric acid, the step of adding to saidhydrocarbons, immediately'prior to their distillation, a small amount ofan alkali metal salt of a phenol,

2. In the distillation of alkylate which has been contacted withsulfuric acid, the step of adding thereto, immediately prior to saiddistillation, a small amount of an alkali metal salt of phenol.

V 3. A method in accordance with claim 2 in which the phenol is derivedfrom petroleum.

4. A method in accordance with claim 2 in which from .005 to .5% byvolume of an alkali metal salt of a phenol is added to the alkylate.

7 DAVID C. WALSH, JR.

ELZA Q. CAMP.

